1. Field of the Invention
The present invention relates to a thermoplastic elastomer composition having an excellent fusion bond property and also to a formed composite product making use of the composition.
More specifically, the present invention pertains to a thermoplastic elastomer composition which is indispensable for the production of a formed composite product composed of two pieces of formed bodies fusion-bonded to each other, one being made of a synthetic resin such as an engineering plastic having excellent mechanical strength and the other being made of the thermoplastic elastomer composition having excellent fusion bond property to the formed one. The present invention is also concerned with a formed composite product produced using the thermoplastic elastomer composition.
2. Description of the Related Art
Engineering plastics having excellent mechanical strength are employed in machine parts and structural parts, which are both used under load. As examples of such engineering thermoplastics, ABS resins, polyamides, polyacetals, polycarbonates, modified polyphenylene oxides, polybutylene terephthalate, polysulfones and wholly aromatic polyesters are known.
On the other hand, molded or otherwise formed (hereinafter collectively referred to a "formed" for the sake of brevity) products of thermoplastic elastomers (TPEs) are produced particularly by high-productivity injection molding and are also used for a variety of applications.
As is well known, a thermoplastic elastomer (TPE) is a material which exhibits properties as vulcanized rubber at room temperature but can be plasticized at high temperatures and formed by plastic processing machines. Its molecule contains both flexible units (soft segments) having rubber elasticity and molecule-restraining units (hard segments) for the prevention of plastic deformation. Examples of such thermoplastic elastomers (TPEs) known to date include styrene-butadiene elastomers, polyolefin elastomers, polyurethane elastomers, vinyl chloride elastomers and acrylic acid ester elastomers.
Injection molding described above permits production of molded articles even of complex shapes and is suited for mass production. It is also applied to the engineering plastics described above, whereby various kinds of machine and structural parts are manufactured efficiently.
There have recently been a strong demand for further improvements in the performance and function of parts and members made of synthetic resins (plastics). Under these circumstances, there is a move toward combining synthetic resins such as the above engineering plastics with thermoplastic elastomers in attempts to form composite products. For this purpose, it is most effective to fusion-bond these two materials together into a composite product by using injection molding which is a molding process common to them.
In general, however, thermoplastic elastomers (TPE) having excellent rubber elasticity have inferior fusion bond property to thermoplastic resins such as engineering plastics and therefore a firm bond cannot be obtained therebetween.
For this reason, when a thermoplastic resin, such as an engineering plastic, and a thermoplastic elastomer (TPE) are to be combined together to produce a formed composite product of high added value such as a composite part and composite member, it is inevitable under the current circumstances to adopt an extremely unproductive bonding process, for example to form complementary concave-convex portions in joint areas of members of the two different materials and then to join them together mechanically, or to apply an adhesive to the joint areas of the members of the two different materials and then to bond them together.
As has been described above, the conventional techniques of forming a composite product from a thermoplastic resin such as an engineering plastic and a thermoplastic elastomer (TPE) are accompanied by many drawbacks such as low efficiency and poor strength and sealing the joint areas.
More specifically, the method in which the two members of the different materials are mechanically joined at the engagement portions, such as complementary concave-convex inter-fitting, has the problems that it requires dies of more complex structures and, depending upon the structure, more production steps. Depending on the final shape of a formed composite product, it may be impossible to attain a strong composite structure. Furthermore, in the case of a formed composite product which is required to be waterproof, for example, water goggles in each of which a glass portion is made of a rigid engineering plastic material and a frame portion that comes into contact with a user's face is made of a soft, high-elasticity thermoplastic elastomer (TPE), no satisfactory water-proofness can be attained by mechanical engagement.
The method which uses an adhesive requires additional steps including application of the adhesive, and is accompanied by the problem that the bonding strength is low because of deterioration of the adhesive or the like.